This invention relates to a mirror projection system for use in a step-and-scan lithographic projection apparatus for imaging a mask pattern, present in a mask, on a substrate by means of a beam of EUV radiation. The beam has a circular segment-shaped cross-section. The projection system is constituted by six imaging mirrors having ordinal numbers 1-6 from the object side to the image side, the second, the third and the sixth mirrors being concave and the fifth mirror being convex.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern on a number of areas of a substrate, which apparatus comprises such a mirror projection system.
EP-A 0 779 528 describes a mirror projection system for use in a step-and-scan lithographic apparatus with which an integrated circuit, IC, mask pattern is imaged on a number of areas of a semiconductor substrate, using EUV radiation. EUV, extreme ultraviolet, radiation is understood to mean radiation having a wavelength in the range between several nm and several tens of nm. This radiation is also referred to as soft X-ray radiation. The use of EUV radiation provides the great advantage that extremely small details, of the order of 0.1 xcexcm or less, can be imaged satisfactorily. In other words, an imaging system in which EUV radiation is used has a very high resolving power without the numerical operator, NA, of the system having to be extremely large, so that also the depth of focus of the system still has a reasonably large value. Since no suitable material is available to make lenses for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional lens projection system.
The lithographic apparatuses currently used in the production of ICs are stepping apparatuses. In these apparatuses, a full field illumination is used, i.e. all areas of the mask pattern are illuminated simultaneously and these areas are simultaneously imaged on one IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area will be positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate of the mask pattern are illuminated. As is known, it remains desirable to have ICs with an increasing number of components.
It is attempted to meet this desire not only by reducing the dimensions of these components but also by enlarging the surface areas of the ICs. This means that the, already relatively highly NA of the projection lens system must be further increased and, for a stepping apparatus, the image field of this system must also be further increased. This is practically impossible.
It has therefore been proposed to change from a stepping apparatus to a step-and-scan apparatus. In such an apparatus, a rectangular or circular segment-shaped sub-area of the mask pattern and hence also such a sub-area of an IC area of the substrate is illuminated, and the mask pattern and the substrate are moved synchronously through the illumination beam, taking the magnification of the projection system into account. A subsequent circular segment-shaped sub-area of the mask pattern is then imaged each time on a corresponding sub-area of the relevant IC area on the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of a subsequent IC area is introduced into the projection beam and the mask is set to its initial position, whereafter said subsequent IC area is scan-illuminated via the mask pattern. This scan-imaging method may be used to great advantage also in a lithographic apparatus in which EUV radiation is used as projection radiation.
The embodiment of the projection system described in EP 0 779 528, intended for use with EUV radiation having a wavelength of 13 nm has, an NA of 0.20 at the image side. The annular image field has an inner radius of 29 mm and an outer radius of 31 mm and a length of 30 mm. The resolution of the system is 30 nm and the aberrations and distortions are sufficiently small to form a good image of a transmission mask pattern on an IC area of a substrate by way of a scanning process. The first and the fourth mirror of this projection system are concave. A first pair of mirrors, consisting of the first and the second mirror, constitutes a magnified image of the object or the mask pattern. This image is transported by a second pair of mirrors, constituted by the third and the fourth mirror, and presented to a third pair of mirrors, constituted by the fifth and the sixth mirror, which provides the desired telecentric image with the required aperture NA=0.20. In this projection system, an intermediate image is formed between the third and the fourth mirror, and the total projection system has a positive magnification.
When such a mirror system is used in a step-and-scan lithographic projection apparatus, a positive magnification means that the mask and the substrate must move in the same direction during scanning. Since both the mask and the substrate are accommodated in a relatively heavy holder, which in its turn forms part of an even heavier displacement table, two heavy masses must be moved in the same direction during scanning, so that stability problems may arise. However, since position accuracies of the order of nm are required in said lithographic apparatus, this apparatus must be extremely stable.
It is an object of the present invention to provide a novel concept for a projection system of the type described in the opening paragraph, with which, inter alia, the above-mentioned problem can be solved. To this end, the projection system is characterized in that the first and the fourth mirror are convex.
In the novel projection system, the formation of an intermediate image is deliberately avoided so that the magnification is negative. When using this system in a step-and-scan apparatus, the mask and the substrate move in opposite directions during scanning. This projection system comprises a first group, constituted by the first and the second mirror, having a collimator function, and a second group, constituted by the third, the fourth, the fifth and the sixth mirror, providing the ultimate image. A further advantage of the projection system is that it is reasonably insensitive to tilts of the mirrors because mainly parts of the mirror sections located proximate to the optical axis are used.
It is to be noted that U.S. Pat. No. 5,686,728 describes a six-mirror projection system for a step-and-scan apparatus. However, this projection system is designed for wavelengths in the range between 100 nm and 300 nm, i.e. not for EUV radiation. Moreover, the mirrors, viewed from the mask to the substrate, are consecutively convex, concave, convex, concave, convex and concave, and the system has a positive magnification.
Within the above-mentioned design concept of the novel projection system, there is still some freedom of choice of the parameters NA, magnification and size of the image field.
A first embodiment of the projection system is characterized in that one of the mirrors is spherical and the other mirrors are aspherical, and in that the system has a numerical aperture of the order of 0.13 to 0.15 at the image side, a magnification M=xe2x88x920.25 and a circular segment-shaped image field having a width of 1 mm.
This projection system is suitable for imaging details having a size of the order of 70 nm.
An aspherical surface is understood to mean a surface whose fundamental shape is spherical but whose actual surface locally deviates from this fundamental shape so as to correct aberrations of the system.
However, the projection system is preferably further characterized in that all mirrors are aspherical, and in that the system has a numerical aperture of the order of 0.20 at the image side, a magnification M=xe2x88x920.25 and a circular segment-shaped image field having a width of 2 mm.
By making all mirrors aspherical, the system can be corrected for a wider image field and the numerical aperture can be increased. Details having a size of the order of 50 nm can be imaged with this system.
The projection system is preferably further characterized in that it is telecentric at the image side.
Consequently, no magnification errors can occur upon undesired displacements of the substrate along the optical axis.
The projection system may be further characterized in that the edge of the fifth mirror provides a boundary for the beam, which boundary substantially has the shape of a circular segment and functions as a diaphragm.
The projection system may be used for imaging both a transmissive mask pattern and a reflective mask pattern. For EUV radiation, a reflective mask pattern can be manufactured more easily than a transmissive mask pattern.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern, present in a mask, on a number of areas of a substrate, which apparatus comprises an illumination unit with a source for EUV radiation, a mask holder for accommodating a mask, a substrate holder for accommodating a substrate, and a projection system. This apparatus is characterized in that the projection system is a mirror projection system as described above.
These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.